Two-Lever Design Rule for High-Entropy Oxide Oxygen Carriers: Minimize Spin Polarization, Maximize Fe-O-Ni Covalency

What is missing in high-entropy oxide (HEO) oxygen carrier design is an actionable rule that links composition to redox performance. We advance a two-lever principle─decrease spin polarization and increase Fe-O-Ni covalency─that lowers the oxygen-vacancy formation energy at the targeted Fe-O-Ni-linked sites and improves CO space-time yield (STY) by 4.7 times. Guided by this rule, we prepare compositionally diverse HEOs and benchmark them in chemical looping reverse water-gas shift (CL-RWGS). Spectroscopy and temperature-programmed reduction indicate that strengthening the Fe-O-Ni covalency increases the fraction of labile lattice oxygen, while electronic-structure calculations connect suppressed spin polarization to lower oxygen vacancy formation energy across representative local environments. A FeMgAlNiZn HEO following the rule achieves a STY of 8.6 mmol_CO·kg_cat^-1·s^-1 under CL-RWGS at 650 °C, substantially outperforming FeMgCoZnMn HEO that violates the rule, and maintains performance over extended cycling. The combined experimental-computational evidence establishes a mechanism-anchored, composition-level guideline for HEO oxygen carriers: avoid cations with a high spin state shift to maintain phase stability, promote Fe-O-Ni linkages to enhance covalency, and tune the electronic structure to minimize spin polarization. This design framework enables rapid, rational navigation of the vast HEO space for carbon-efficient CO₂ to CO conversion.